What is Nectar necessary for, and how can I replenish my reserves of it?

Flowers need Nectar to grow and bloom. The resource is also necessary for hybridization and running the mill.
A unit of Nectar takes 3 minutes to restore. You can also replenish your reserves of it by using different options from the Nectar section in the Shop. Lastly, a certain amount of nectar can be obtained by gathering collectible items.